This invention relates to a semiconductor device using a silicon nitride film or silicon oxide film containing nitrogen as a gate insulating film and a method for manufacturing the same.
In order to enhance the performance of a semiconductor integrated circuit including MIS semiconductor elements and lower the cost thereof, it is important to miniaturize the elements and increase the integration density. Miniaturization of the elements is effected according to the design rule.
Further, in order to increase the integration density of the elements, it is important to not only reduce the size of the elements but also reduce the size of the element isolating region. As the effective technique for miniaturizing the size of the element isolating region, a trench type element isolation (STI: Shallow Trench Isolation) technique is known.
When a MOS transistor using a polysilicon film containing boron as a gate electrode is miniaturized, it is necessary to use a nitrided silicon oxide film (which is a silicon oxide film containing nitrogen) as a gate insulating film in order to prevent boron from being diffused into the silicon substrate. It is necessary to simultaneously supply an oxidizing agent and nitriding agent in order to form a thinner nitrided silicon oxide film.
Next, a method for manufacturing the MOS transistor using the above gate insulating film (nitrided silicon oxide film) is explained with reference to FIGS. 9A to 9H. These figures show cross sections of the MOS transistor taken along a line passing across the gate electrode and set in parallel to the gate length direction.
First, as shown in FIG. 9A, a silicon oxide film 82 with a thickness of 10 nm is formed on the (100) surface of a silicon substrate 81 by use of a thermal oxidation method. Then, as shown in FIG. 9A, a silicon nitride film 83 with a thickness of 200 nm is formed on the silicon oxide film 82 by use of the LPCVD method.
Next, as shown in FIG. 9B, trench type shallow element isolation grooves 84 are formed in the surface of the silicon substrate 81 by sequentially etching the silicon nitride film 83, silicon oxide film 82 and silicon substrate 81.
In more detail, a photoresist pattern (not shown) which defines an element forming region (active area) is formed on the silicon nitride film 83 and the pattern of the photoresist pattern is transferred onto the silicon nitride film 83 by etching the silicon nitride film 83 by use of an RIE method with the photoresist pattern used as a mask.
Next, after the photoresist pattern is removed, the element isolation grooves 84 are formed by sequentially etching the silicon oxide film 82 and the silicon substrate 81 by use of the RIE method with the silicon nitride film 83 used as a mask.
After this, as shown in FIG. 9C, a silicon oxide film 85 with a thickness of 15 nm is formed on the exposed surface of the silicon substrate 81 by use of a thermal oxidation method.
Next, as shown in FIG. 9D, an element isolation insulating film 86 is filled in the internal portions of grooves formed by the element isolation grooves 84 as well as the silicon nitride film 83 and silicon oxide film 82 lying thereon, and then the surface of the structure is made flat.
In more detail, a silicon oxide film used as the element isolation insulating film 86 is formed on the entire surface by use of the LPCVD method so as to fill the grooves formed of the element isolation grooves 84 and the silicon nitride film 83 lying thereon, then the silicon film is polished by use of the CMP method until the surface of the silicon nitride film 83 is exposed. As a result, the structure as shown in FIG. 9D is obtained.
Next, as shown in FIG. 9E, the element isolation insulating film (silicon oxide film) 86 is retreated to substantially the surface portion of the silicon substrate 81 by use of an ammonium fluoride solution and the silicon nitride film 83 is removed by use of a hot phosphoric acid, then the silicon oxide film 82 is removed by use of a dilute hydrofluoric acid to expose the surface of the silicon substrate 81 (active area) in the element forming region.
Next, for example, as shown in FIG. 9F, an oxidizing-nitriding process is effected at 850xc2x0 C. by use of dinitrogen monoxide gas, to form a nitrided silicon oxide film (gate insulating film) 87 with a thickness of 4 nm on the exposed surface of the silicon substrate 81 and then an amorphous silicon film 88 with a thickness of 100 nm, which contains boron as impurity with high impurity concentration and which will be used as a gate electrode, is formed by use of the LPCVD method.
After this, like a conventional MOS transistor manufacturing method, the processes for patterning the gate electrode, for forming source and drain diffusion layers and for forming wirings are effected to complete a MOS transistor.
However, this type of MOS transistor manufacturing method has the following problem.
As shown in FIG. 9G, the portion of the nitride silicon oxide film (gate insulating film) 87 lying on the upper-end corner portion of the side wall of the element isolation groove 84 is not so thick as the portion of the same lying on the element forming region. Therefore, the breakdown voltage of the nitrided silicon oxide film (gate insulating film) 87 on the upper-end corner portion of the side wall of the element isolation groove 84 in which the electric field is concentrated becomes low and thus the reliability is lowered.
Further, in the thermal oxidation method, which is a conventional method for forming a normal gate insulating film, the oxidation rate varies on the (100) face and the (110) face which correspond to the side wall surface of the element isolating groove 84, (the rate on the (110) face is about 1.5 times greater than that on the (100) face) and therefore, as shown in FIG. 9H, the film thickness of the thermal silicon oxide film (gate insulating film) 87 at the upper-end corner portion of the side wall of the element isolation groove 84 is greater than the film thickness of the thermal silicon oxide film (gate insulating film) 87 in the element forming region. A high breakdown voltage of the thermal silicon oxide film (gate insulating film) 87 is therefore attained and thus there have been no problems regarding a low reliability due to a low gate breakdown voltage. However, if a dinitrogen monoxide gas is employed, both oxidation and nitration may occur, and thus the difference of oxidation rate on the (100) face and the (110) face may be reduced.
In order to solve the problem of a lowering in the breakdown voltage of the nitrided silicon oxide film (gate insulating film) 87, a portion of the nitrided silicon oxide film (gate insulating film) 87 at the above-mentioned corner portion, that is, a portion lying between the upper-end corner portion of the side wall of the element isolation groove 84 and the end portion of the element forming region formed in contact therewith may be made thicker than a portion of the nitrided silicon oxide film (gate insulating film) 87 which lies on the central flat portion of the element forming region.
Such a technique for forming a gate insulating film having different film thickness on different regions of the substrate is known in the prior art (refer to Japanese Patent Application No. 3-249810).
However, if this type of conventional technique is used, it will be necessary to use the photolithography in order to mask the corner portion, and therefore, additional steps (additional time for effecting the steps) and additional manufacturing costs will be necessary.
Further, a problem of misalignment may occur and it is therefore difficult to form a thick gate insulating film on the corner portion without fault.
As described above, in a MOS transistor using the nitrided silicon oxide film as the gate insulating film, it has been considered that a nitrided silicon oxide film is formed so as to have a thin part on the central flat portion of the element forming region as designed and a thick part on the corner portion, to prevent a lowering in the breakdown voltage of the gate insulating film in the corner portion.
However, such a conventional method for forming the gate insulating film having different film thickness on the different regions of the substrate requires the photolithography.
Therefore, there occur problems that additional steps (additional time for effecting the steps) as well as additional manufacturing costs will be needed and it is difficult to form a thick gate insulating film on the corner portion without fault, due Lo misalignment.
This invention has been achieved in consideration of the above problems. An object of the present invention is to provide a semiconductor device having a gate insulating film which is formed of a silicon nitride film or silicon oxide film containing nitrogen, which is formed in a self-alignment manner, and which can prevent deterioration of the breakdown voltage in a boundary region defined between an element forming region and an element isolating region, and to provide a method for manufacturing the same.
According to the first aspect of the present invention, there is provided a semiconductor device comprising a silicon substrate including an element forming region, an element isolating region, and a boundary region defined between the element forming region and the element isolating region, including the boundary between the element forming region and the element isolating region, and a gate insulating film formed on the surface of the silicon substrate to extend from the element forming region to the element isolating region across the boundary region, wherein the gate insulating film includes either of a silicon nitride film or a silicon oxide film containing nitrogen and is formed in a self-alignment manner to make the thickness of the gate insulating film on the boundary region greater than the thickness of the gate insulating film in the regions other than the boundary region.
According to the second aspect of the present invention, there is provided a method for manufacturing a semiconductor device, comprising the steps of dividing the silicon substrate into the element forming region and the element isolating region, doping nitrogen into the surface of the silicon substrate in the element forming region, and-forming the gate insulating film on the surface of the silicon substrate so that the gate insulating film can extend from the element forming region to the element isolating region across the boundary region, by the heat treatment in an atmosphere containing an oxidizing agent.
According to the third aspect of the present invention, there is provided a method for manufacturing the semiconductor device, comprising the steps of dividing a silicon substrate into an element forming region and an element isolating region, doping nitrogen into the surface of the silicon substrate in the boundary region, and forming a silicon nitride film or a silicon oxide film containing nitrogen as the gate insulating film so that the gate insulating film can extend on the surface of the silicon substrate from the element forming region to the element isolating region across the boundary region.
According to the fourth aspect of the present invention, there is provided a method for manufacturing the semiconductor device, wherein the silicon substrate is a crystalline silicon substrate, and the method comprises the steps of dividing the crystalline silicon substrate into an element forming region and an element isolating region, selectively forming the surface of the silicon substrate in the boundary region into an amorphous form, and forming the silicon nitride film or the silicon oxide film containing nitrogen as the gate insulating film by use of a nitriding method, so that the gate insulating film can extend from the element forming region to the element isolating region across the boundary region.
It is preferable to use an insulating film of a laminated structure including a silicon nitride film as the gate insulating film.
In the methods for manufacturing the semiconductor device according to this invention, it is preferable to perform the step of doping nitrogen into the surface of the silicon substrate by use of, for example, a thermal nitriding method using nitriding agent gas such as nitrogen monoxide gas or ammonium gas, a radical nitriding method using active nitrogen atoms, or an ion implantation method using nitrogen ion.
Further, in the methods for manufacturing the semiconductor device according to this invention, it is preferable to form a chemically grown film on a region other than the boundary region of the element forming region before the gate insulating film is formed by use of the nitriding method.
Further, in the methods for manufacturing the semiconductor device according to this invention, it is preferable to use ion of an inert element such as helium, neon, argon, krypton or xenon, nitrogen ion, oxygen ion or silicon ion as the ion implanted into the surface of the silicon substrate.
According to this invention, in the semiconductor device using the silicon nitride film or silicon oxide film containing nitrogen as the gate insulating film, since the film thickness of the gate insulating film in the boundary region defined between the element forming region and the element isolating region is greater than the film thickness of the gate insulating film in the element forming region, a lowering in the breakdown voltage in the boundary region can be suppressed.
Further, since the gate insulating film can be formed in the self-alignment manner by the manufacturing method of this invention, the number of steps (time for effecting the steps) and the manufacturing cost can be reduced and the film of a sufficient thickness in the boundary region can be stably provided.
According to this invention, since nitrogen is doped into the surface of the silicon substrate in which the film thickness is made small and nitrogen is not doped into the surface of the silicon substrate in which the film thickness is made large, the gate insulating film (silicon oxide film containing nitrogen, silicon nitride film) having different film thickness can be formed in a self-alignment manner by the heat treatment in an atmosphere containing an oxidizing agent.
Further, according to this invention, since nitrogen is not doped into the surface of the silicon substrate in which the film thickness is small and nitrogen is doped into the surface of the silicon substrate in which the film thickness is large, the gate insulating film (silicon oxide film containing nitrogen, silicon nitride film) having different film thickness can be formed in a self-alignment manner by use of the deposition method.
Further, according to this invention, in the area where nitrogen is not doped into the surface of the silicon substrate, the film thickness is made small while in the area where nitrogen is doped into the surface of the silicon substrate, the film thickness is made large. Hence, the gate insulating film (silicon oxide film containing nitrogen, silicon nitride film) having different film thickness can be formed in a self-alignment manner by use of the nitriding method.
Further, according to this invention, since the surface of the silicon substrate in which the film thickness is made small is kept in the single crystal form and the surface of the silicon substrate in which the film thickness is made large is formed into the amorphous form, the gate insulating film (silicon oxide film containing nitrogen, silicon nitride film) having different film thickness can be formed in a self-alignment manner by use of the nitriding method.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.